How it works
A 5G Broadcast receiver has to bootstrap itself from nothing: there's no SIM, no return channel, and no prior connection to the network. LTE-based 5G Broadcast is a one-way physical layer built on the LTE TS 36.xxx series -- a receiver acquires the cell, reads the broadcast control information, then decodes the multicast traffic channel, with no uplink and no per-user state.
Key specifications: ETSI TS 103 720 (5G Broadcast System for linear TV and radio), 3GPP TR 36.976 (LTE-based 5G broadcast overview), 3GPP TS 26.346 (MBMS protocols and codecs), 3GPP TS 26.347 (MBMS application programming interface).
PMCH (Physical Multicast Channel) carries the broadcast payload; in an MBMS-dedicated cell it occupies almost the whole carrier. CAS (Cell Acquisition Subframe) is the entry point -- one radio frame per 40 ms period (SFN mod 4 == 0) uses 15 kHz SCS with the standard cyclic prefix and carries PSS, SSS, PBCH, PCFICH, PDCCH and PDSCH, letting the receiver synchronise and decode MIB-MBMS (on PBCH) and SIB1-MBMS without a USIM. The CAS occupies a narrow 15 or 25 PRB (3 or 5 MHz) regardless of the PMCH bandwidth, so the receiver bootstraps narrow and the SIBs then point it at the wider PMCH. MCCH (Multicast Control Channel) carries the MBSFN (Multicast-Broadcast Single-Frequency Network) area configuration -- which services map to which PMCH, MCS, scheduling -- and is re-read at each modification-period boundary so the receiver can pick up transmitter-side changes. MTCH (Multicast Traffic Channel) is the logical channel that maps the user-service data onto PMCH.
Acquisition order on the receiver, step by step:
- Synchronise on PSS/SSS in the CAS.
- Decode PBCH to get MIB-MBMS (bandwidth, SFN, semi-static CFI).
- Decode SIB1-MBMS (cell access, SI scheduling).
- Decode SIB13 (MBSFN area info, ROM info, extended bandwidth).
- Decode MCCH.
- Decode PMCH/MTCH -- the receiver is now showing the channel.
Numerologies (subcarrier spacings)
The choice of subcarrier spacing (SCS) trades cyclic-prefix length (and therefore maximum SFN reach and mobility tolerance) against overhead. The full set profiled by ETSI TS 103 720 is:
| SCS | 3GPP release | Cyclic prefix (approx.) | Primary use case |
|---|---|---|---|
| 15 kHz | (LTE baseline) | standard | CAS; mixed-mode operation |
| 7.5 kHz | Rel-14 | ~33 microseconds | standard-area broadcast |
| 1.25 kHz | Rel-14 | ~200 microseconds | high-power high-tower, very large SFN |
| 2.5 kHz | Rel-16 | ~100 microseconds | mobile reception (up to ~250 km/h) |
| 0.37 kHz | Rel-16 | ~300 microseconds | very large SFN (inter-site distance up to ~100 km) |
The 0.37 kHz numerology uses a 3 ms slot structure and defines two reference-signal placement
variants selected by timeSeparation (sl2 and sl4);
this is signalled per MBSFN area in MBSFN-AreaInfo-r16 (TS 36.331). The MBSFN
reference-signal patterns for each numerology are defined in
TS 36.211 clause 6.10.2.2.
Channel bandwidths
The PMCH can use the standard LTE bandwidths (up to 20 MHz) carried in MIB-MBMS, or the UHF
broadcast bandwidths added in Release 17 and signalled by pmch-Bandwidth-r17 in
SIB13:
| pmch-Bandwidth-r17 | Bandwidth | PRB |
|---|---|---|
n30 | 6 MHz | 30 |
n35 | 7 MHz | 35 |
n40 | 8 MHz | 40 |
When present, pmch-Bandwidth-r17 overrides the narrowband CAS bandwidth for the
PMCH region; its absence means the legacy LTE bandwidth applies. See
Operational Parameters in Use for the
values observed in practice.
Specifications by 3GPP release
The features below are profiled by ETSI TS 103 720. For the standards-tracking narrative with the version-to-release mapping and the release-by-release table, see Standards: 5G Broadcast - TV and Radio Services.
Release 14 (FeMBMS baseline). The foundation: the MBMS-dedicated cell
(almost 100% of the carrier for broadcast), the 1.25 kHz and 7.5 kHz numerologies with
extended cyclic prefix, the CAS frame structure, 256-QAM on PMCH (with the extended TBS
table), and the broadcast control blocks MIB-MBMS (MasterInformationBlock-MBMS-r14,
on PBCH) and SIB1-MBMS (SystemInformationBlockType1-MBMS-r14). Shorter MCCH
repetition and modification periods (down to one radio frame) support faster
system-information updates.
Release 16 (LTE-based 5G Terrestrial Broadcast, normative). The normative
work item adds the 2.5 kHz and 0.37 kHz numerologies and their reference-signal patterns,
PDCCH Format 4 (aggregation level 16) for robust control-channel reception, PBCH repetition
across CAS subframes for faster MIB acquisition, the MBSFN-AreaInfo-r16 structure
(carrying subcarrierSpacingMBMS-r16 and timeSeparation-r16), the
semi-static CFI in MIB-MBMS (semiStaticCFI-MBMS-r16), and Receive-Only Mode (ROM)
information in SIB13 (MBMS-ROM-Info-r16), which lets a receiver be redirected to
MBMS content on another carrier without a return channel.
Release 17 (channel bandwidths). Adds the 6/7/8 MHz PMCH bandwidths (30/35/40
PRB) via pmch-Bandwidth-r17, with updated TBS and PMCH resource-allocation
tables. This is the release profiled by the published ETSI TS 103 720 v1.2.1 (June 2023),
which also defines the Base, Main and 5GMS receiver categories.
Release 18 (UHF bands, receive-only). A spectrum-layer change only: the receive-only (downlink-only) UHF bands Band 107 (612 to 652 MHz) and Band 108 (470 to 698 MHz), defined as standalone downlink-only (SDO) bands in Table 5.5H-1 of TS 36.101, with base-station requirements in TS 36.104; TS 36.307 makes the SDO broadcast bands release-independent from Release 17. There are no new numerologies and no MAC or RRC changes. The published ETSI TS 103 720 v1.2.1 does not yet include specific frequency bands; its clause 8 defers them to a future revision.
Release 19 (PMCH Phase 2, CAS muting, new bands). PMCH Phase 2 adds time
interleaving (spreading a transport block across N consecutive subframes for time diversity,
with a scheduling window M), frequency interleaving, a per-subframe cyclic shift (different
values per cell in an SFN reduce boundary interference), TBS scaling, and extended SI
scheduling periods, all carried in a new RRC v1900 extension chain. The PMCH-specific MCS
tables (TS 36.213 clause 11.1, Tables
11.1-1 and 11.1-2, the latter extending to 256-QAM) already exist in the Release 18 version
of TS 36.213. CAS muting frees synchronisation-channel airtime for PMCH by transmitting
PSS/SSS/PBCH only in the active part of each period (cas-MutingConfig-r19 in the
new SystemInformationBlockType1-MBMS-v1900 extension). Two further receive-only
UHF bands, Band 112 (470 to 608 MHz) and Band 113 (606 to 698 MHz), are defined in Table
5.5H-1 of TS 36.101 and are release-independent from Release 17. These features postdate the
published ETSI TS 103 720 v1.2.1 and are expected in a later revision.
5G-MAG's own reference tools overview slide deck summarises the transmitter and receiver chain. See also 5G Media Streaming over eMBMS (3GPP Release 17) for the related VideoTech session.
Related: 5G Broadcast - Emergency Alerts (Cell Broadcast Service public warning, layered on this same transmit and receive chain) · 5G Multicast Broadcast Services (MBS) (the NR-native multicast and broadcast technology, a separate topic with a similar name) · Standards: Content Delivery Protocols (the FLUTE and ROUTE transport used to carry files over the broadcast bearer) · Standards: DVB-I Services over 5G Systems (DVB-I service discovery, used to present broadcast services alongside broadband ones)